Fix getmntent check with -Werror.

[gonsolo]

Fix a warning. The test is just compile tested and the return value is not used anyway.
Also, in my private build I compile with "-Werror" and without this patch this fails.

It also fails to build:
../libs/pbd/mountpoint.cc:114:10: fatal error: sys/ucred.h: Datei oder Verzeichnis nicht gefunden 114 | #include <sys/ucred.h> | ^~~~~~~~~~~~~ compilation terminated.

[gonsolo]

The test program cannot be run. It always fails with a segmentation fault. A better one would be:

FILE *f = setmntent("/", "r");
if (f) { endmntent(f); }

[x42]

The test is not executed, but it needs to compile without warning if OP wants to use -Werror.

[kiilerix]

The current check script will however compile even if mntent.h doesn't contain getmntent. The default declaration of getmntent is fine.

The check could thus be simplified to
msg="Checking for mntent.h", fragment = "#include <mntent.h>",
which I assume also works fine for the use case of -Werror.

A more (pointless) thorough check that checks for valid content of mntent.h could perhaps also do
struct mntent *tmp = getmntent(0);

[luzpaz]

Is this still relevant ?

[gonsolo]

Yes.

The ifdef in libs/pbd/mountpoint.cc is just a safety check to make things clear.

The test in libs/pbd/wscript is bogus:

1. It is never run, just compiled to check whether getmntent is available. When run it always fails with a segmentation violation.
2. It casts a pointer (size 8) to an int (size 4) which causes a warning, which causes the check to fail when built with "-Werror" like my branch (https://github.com/gonsolo/ardour/tree/gonsolo).

So either

1. keep the bogus check, since it is not run, but don't cast to int, or
2. use setmntent like I said in an earlier comment.

@kiilerix: Just using the include doesn't work, I just checked.

I once suggested to add -Werror to Ardour but it was rejected. I like it because the build compiles cleanly without warnings, ergo every new warning clearly stands out, all the suppressed warnings could be removed one after another to get a cleaner repository.

Dropping this patch and adding -Werror leads to a mysterious error:

missing #include <sys/ucred.h>

So while it's not strictly necessary my suggestion would be just pull this request and be done with it.
What do you think, @luzpaz @x42?